1. Field of the Invention
The present invention relates to a thermal printhead for recording on a recording medium such as a paper thermosensitively or by thermal transfer. The present invention also relates to a method of making such a thermal printhead.
2. Description of the Related Art
FIGS. 9 through 11 illustrate a conventionally used thick film type thermal printhead. The thermal printhead Xxe2x80x2 includes a substrate 1xe2x80x2 and a heat sink plate 2xe2x80x2 each of which is in the form of an elongated rectangle
The substrate 1xe2x80x2 is provided with a glaze layer 3xe2x80x2 formed thereon. The glaze layer 3xe2x80x2 serves as a heat-retaining layer. The glaze layer 3xe2x80x2 is formed with a common electrode 4xe2x80x2, a plurality of individual electrodes 5xe2x80x2 and a heating resistor 6xe2x80x2. Further, a plurality of drive ICs 8xe2x80x2 are mounted on the glaze layer 3xe2x80x2.
The common electrode 4xe2x80x2 comprises a plurality of comb-teeth 41xe2x80x2 and a common line 40xe2x80x2 connected to one end of each comb-tooth 41xe2x80x2. Each of the individual electrodes 5xe2x80x2 has one end 5axe2x80x2 extending between two adjacent comb-teeth 41xe2x80x2.
The heating resistor 6xe2x80x2 extends across the comb-teeth 41xe2x80x2 and the respective ends 5axe2x80x2 of the individual electrodes 5xe2x80x2.
The common electrode 4xe2x80x2, the individual electrodes 5xe2x80x2 and the heating resistor 6xe2x80x2 are protected by an insulating protective layer 60xe2x80x2. The insulating protective layer 60xe2x80x2 may be made of glass for example and is transparent. Therefore, it is possible to view the common electrode 4xe2x80x2, the individual electrodes 5xe2x80x2 and the heating resistor 6xe2x80x2 through the insulating protective layer 60xe2x80x2.
The other end 5bxe2x80x2 of each individual electrode 5xe2x80x2 is connected to the corresponding drive IC 8xe2x80x2 via a wire W1xe2x80x2. The drive ICs 8xe2x80x2 are protected by a hard coating layer 81xe2x80x2.
The substrate 1xe2x80x2 is bonded to the heat sink plate 2xe2x80x2. As clearly shown in FIG. 12, the bonding is performed by forming an adhesive layer 20xe2x80x2 on the heat sink plate 2xe2x80x2 and pressing the substrate 1xe2x80x2 onto the adhesive layer.
In bonding, it is necessary to precisely position the substrate 1xe2x80x2 relative to the heat sink plate 2xe2x80x2 so that the heating resistor 6xe2x80x2 is positioned in accordance with the customer""s specifications. For positioning the substrate 1xe2x80x2 relative to the heat sink plate 2xe2x80x2, use is made of a positioning apparatus (not shown) for example. The positioning apparatus comprises a fixing base for fixing the heat sink plate 2xe2x80x2, an image pick-up device 91 for detecting the absolute position of the heat sink plate on the fixing base, and a monitor 93 for displaying the image taken by the image pick-up device.
The monitor has a display provided with an X-reference line X0xe2x80x2 extending longitudinally of the heat sink plate 2xe2x80x2 and a Y-reference line Y0xe2x80x2 extending widthwise of the heat sink plate 2xe2x80x2. The operator positions the substrate 1xe2x80x2 while watching, via the display of the monitor, the image taken by the image pick-up device. For example, the operator moves the substrate 1xe2x80x2 manually so that the heating resistor 6xe2x80x2 overlaps the X-reference line X0xe2x80x2, whereas the end 5axe2x80x2 of a selected individual electrode 5xe2x80x2 (or a selected comb tooth 41xe2x80x2) overlaps the Y-reference line Y0xe2x80x2.
As shown in FIG. 14, the thermal printhead Xxe2x80x2 may be formed with a conductive protective layer 61xe2x80x2. The conductive protective layer 61xe2x80x2 covers the heating resistor 6xe2x80x2 via the insulating protective layer 60xe2x80x2. The conductive protective layer 61xe2x80x2 is provided to prevent electrostatic breakdown of the heating resistor 6xe2x80x2. The conductive protective layer 61xe2x80x2 may be formed of a material containing carbon black for example to provide conductivity. In this case, the conductive protective layer 61xe2x80x2 is black and opaque for example.
Thus, in the thermal printhead X as shown in FIG. 14, the common electrode 4xe2x80x2, the ends 5axe2x80x2 of the individual electrodes 5xe2x80x2 and the heating resistor 6xe2x80x2 cannot be viewed because they are covered with the conductive protective layer 61xe2x80x2 which is opaque. Therefore, as shown in FIG. 15, the common electrode 4xe2x80x2, the individual electrodes 5xe2x80x2 and the heating resistor 6xe2x80x2 cannot be viewed on the display of the monitor of the positioning apparatus. As a result, with the above-described positioning apparatus, it is not possible to position the substrate 1xe2x80x2 relative to the heat sink plate 2xe2x80x2 by referring to the positions of the heating resistor 6xe2x80x2 for example.
It is, therefore, an object of the present invention to precisely position a substrate relative to a heat sink plate in making a thermal printhead having a heating resistor provided on the substrate and covered with an opaque conductive protective layer.
In accordance with a first aspect of the prevent invention, there is provided a thermal printhead comprising an elongated rectangular substrate including an attaching surface and a non-attaching surface, and a heat sink plate attached to the attaching surface of the substrate, the non-attaching surface of the substrate being provided with a common electrode, a plurality of individual electrodes, a heating resistor extending longitudinally of the substrate in conduction with the common electrode and the individual electrodes, an insulating protective layer and an opaque conductive protective layer for covering the heating resistor. The thermal printhead further includes a positioning indicia which serves as a reference for positioning the substrate relative to the heat sink plate.
Preferably, the positioning indicia may be provided on the non-attaching surface of the substrate at a portion avoiding the conductive protective layer.
Preferably, the positioning indicia may include a first positioning reference portion extending longitudinally of the substrate and a second positioning reference portion extending widthwise of the substrate. The positioning indicia may comprise a cross for example.
Preferably, one of the first and the second positioning reference portions may be formed of the same material as that of the heating resistor, whereas the other one of the first and the second positioning reference portions may be formed of the same material as that of the common electrode and the individual electrodes.
Preferably, the positioning indicia may comprise a plurality of positioning marks. One of the positioning marks may be arranged at one end of the substrate, whereas another of the positioning marks may be arranged at the other end of the substrate.
In accordance with a second aspect of the present invention, there is provided a method of making a thermal printhead comprising an elongated rectangular substrate including an attaching surface and a non-attaching surface, and a heat sink plate attached to the attaching surface of the substrate, the non-attaching surface of the substrate being provided with a common electrode, a plurality of individual electrodes, a heating resistor in conduction with the common electrode and the individual electrodes, an insulating protective layer and an opaque conductive protective layer for covering the heating resistor, and a positioning indicia including a first positioning reference portion having a first positioning reference line and a second positioning reference portion having a second positioning reference line. The method comprises the steps of forming the common electrode and the plurality of individual electrodes, forming the heating resistor, positioning the substrate relative to the heat sink plate, and attaching the substrate onto the heat sink plate. The step of positioning the substrate utilizes an image pick-up device for picking up an image of the positioning indicia and a monitor having a display on which a first reference line and a second reference line are set. The substrate is so moved that the first and the second positioning reference lines imaged on the monitor coincide with the first and the second reference lines, respectively.
Preferably, the first reference line extends longitudinally of the substrate, and the second reference line extends widthwise of the substrate. The first reference line and the second reference line intersect at right angles.
Preferably, the positioning indicia may include a first positioning mark and a second positioning mark. The first positioning mark may be arranged at one end of the substrate, and the second positioning mark may be arranged at the other end of the substrate. The positioning step may include positioning the substrate relative to the heat sink plate with reference to the first positioning mark and thereafter positioning the substrate relative to the heat sink plate with reference to the second positioning mark.
Preferably, the first positioning reference portion may be formed in the step of forming the electrodes, and the second positioning reference portion may be formed in the step of forming the heating resistor. The entirety of the positioning indicia may be formed at once in the step of forming the electrodes. Alternatively, the entirety of the positioning indicia may be formed at once in the step of forming the heating resistor.